PSCAD Simulation Model of D-Statcom for Voltage Sag Improvement

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International Journal of Computer Applications (0975 – 8887)

Volume 59 – No.2, December 2012

45

PSCAD Simulation Model of D-Statcom for

Voltage Sag Improvement

Tamal RoyCSVTU, Bhilai

SSCET, Junwani, Bhilai

Mahesh SinghCSVTU, Bhilai

SSCET, Junwani, Bhilai

ABSTRACT

This paper deals with the modeling and simulation of

Distribution Static Compensator (D-STATCOM) for voltagesag mitigation. The D-STATCOM simulation model consists

of 12-pulse inverter, which is connected in shunt with thesystem. A generalized sinusoidal pulse width modulation(SPWM) technique was developed in the proposed controller

for fast control action of the D-STATCOM. Simulations of

the D-STATCOM were carried out by using Power SystemComputer Aided Design (PSCAD) software. Simulationresults prove that the D-STATCOM is capable of mitigatingvoltage sag as well as improving power quality of a

distribution system.

Keywords

Power quality, DSTATCOM, SPWM, voltage sag.

1. INTRODUCTION

An electric distribution system is a part of an electric system between the bulk power sources and the end users. The bulk

power sources are located far away from the load centre andare served to the consumer via distribution system. Modernindustries involve the use of non-linear and electronically

switched devices in distribution systems. These devices

results in power quality problems like voltage sag, voltageswell, flicker, harmonics, voltage imbalance etc. This maycause an impact to the electric supply thus may affect themanufacturing industry and impede the economic

development in a country [1].

As a result, development of compensator for improving powerquality has become increasing concern for the researchers forthe past few decades. Passive compensators like shunt reactor

and capacitor are uncontrolled devices and are not able to

control the continuous variation of parameters in power

distribution system. This leads to the development of new andfast compensator. Just like FACTS devices used fortransmission control, custom power devices were developed

for distribution control. The custom power devices include

compensators like DSTATCOM, DVR, Unified Series ShuntCompensator (USSC), Battery Energy Storage system (BESS)and many other such controllers [2]. Since the introduction of

FACTS and custom power concept, devices such as UnifiedPower Flow Controller(UPFC), Synchronous StaticCompensator(STATCOM), Dynamic Voltage Restorer(DVR),

solid-state transfer switch and solid-state fault current limiter

are developed for improving power quality and reliability of asystem[3]-[5]. Investigations have been carried out to studythe effectiveness of these devices in power quality mitigation

such as sag compensation, harmonics eliminations, unbalancecompensation, reactive power compensation, power flowcontrol, power factor correction and flicker reduction[3],[6]-

[8]. These devices have been developed for mitigating

specific power quality problems.

Voltage sags is the most important power quality problems

faced by many industries and utilities. It contributes more than80% of power quality problems that exist in power system. By

definition, voltage sag is an rms (root mean square) reductionin the AC voltage at the power frequency, for duration from a

half cycle to a few seconds. Voltage sags are not tolerated bysensitive equipments used in modern industrial plants such as

process controllers, programmable logic controllers (PLC),adjustable speed drives and robotics [1].

This paper deals with the modeling and simulation ofDistribution Static Compensator (D- STATCOM) and it’s

capability in mitigating Voltage Sag. The modeling andsimulation of D- STATCOM is carried out using the

PSCAD/EMTDC [9] [10] software.

2. Basic Configuration and Operation of

D-STATCOM

The D- STATCOM is a three phase shunt connected power

electronic device. It is connected near the load at thedistribution systems. The major component of D-STATCOM

is shown in Fig.1. The basic components of D-STATCOM are12-pulse voltage source inverters composed of forcecommutated power semiconductor switches which areconnected in shunt with the line through a set of shunt

injection transformers.

Since the D- STATCOM employs an inverter to convert theDC link voltage Vdc on the capacitor to a voltage source ofcontrollable magnitude and phase, it can be treated as

voltage- controlled source. Fig. 1 shows the inductance L and

resistance R which represent the equivalent circuit elements ofthe step down transformer. Let Vs is the distribution systemvoltage, Vi is the effective output voltage of the D-STATCOM and δ is the power angle. The reactive power

Fig. 1 Basic Building Blocks of D-STATCOM





46

output of the D- STATCOM either inductive or capacitive

depending on the operating mode of the D- STATCOM. Thecontroller of the D- STATCOM is used to operate the inverter

in such a way that the phase angle between the invertervoltage and the line voltage is dynamically adjusted so thatthe D- STATCOM generates or absorbs the desired reactive

power at the point of connection. If V i = Vs, then the reactive

power is zero and the D-STATCOM does not generate orabsorb reactive power. When Vi is greater than Vs, the D-STATCOM shows an inductive reactance connected at itsterminal. The current (I) flows through the transformer

reactance from the D- STATCOM to the ac system, and thedevice generates capacitive reactive power. When

Vs is

greater than Vi, the D- STATCOM shows the system as acapacitive reactance. Then the current flows from the ac

system to the D- STATCOM resulting in the device absorbingreactive power.

3. Simulation Model of D-STATCOM

The simulation model of the D- STATCOM has beendeveloped using PSCAD/EMTDC software is shown in Fig.2.

It consist of 12- pulse inverter arrangement with their primaries connected in series. The first inverter is connected

to the system through a Y-Y arrangement whereas a Y-Δconnection is used for the second inverter. Each inverter

operates as a 6- pulse inverter, with the Y-Δ inverter beingdelayed by 30º with respect to the Y-Y inverter. Fig. 3 alsoshows that the proposed 12- pulse D- STATCOM configuredwith the GTO’s used as a power devices. The GTO’s are

connected anti parallel with diodes for commutation purposesand charging of the DC capacitor. This GTO switchcombination and its capability to act as a rectifier or as aninverter with instantaneous current flows in forward or

direction, respectively, constitutes the basic voltage sourceconverter concept.

The DC side of D- STATCOM is connected in parallel tokeep the voltage on DC side as low as possible and to improveutilization of DC side capacitor. The two set of three single-

phase transformers with a leakage reactance of 0.01 per unitare considered. The first transformer is in Y-Y connection andthe second transformer is in Y-Δ connection. This is to give a30º phase shift between the pulses and to reduce the

harmonics generated from the D- STATCOM. Bothtransformers are stepped down from 22kV to 4.16kV. The D-

STATCOM is connected in shunt with the system. Thecapacitor plays an important role in the D- STATCOM

operation by acting as a DC source to provide reactive powerin the system and to regulate the DC voltage. The size of theDC capacitor considered in this simulation is 3340 µF.

Fig. 2 Simulation Model of D- STATCOM Using 12- Pulse

Inverter

4. Controller Model of D-STATCOM

Fig. 3 Reactive Power Control Loop

The control used in this simulation is AC voltage control. This

control is divided in to two parts, that is, sinusoidal pulsewidth modulation (SPWM) and reactive power control. Fig. 3shows that the PI controller regulates the AC side voltage

sourced converter i.e. reactive power into or out of the voltagesourced converter. The output of the PI controller is the angle

order, which is used to maintain the phase shift. The reactive power flow from the system is compared to the reference per

unit voltage that contributes to a change of the phase shift.The difference of phase shift will provide the necessaryreactive power from the DC capacitor.

I

P V e r r 1

Shft

Vref

Qm

D+

F

-

Tconst1

Pgain1

N

D

N/D

3120.0

N

D

N/D *0.03

TIME *

VrpuMax

D

E0.1

D+

F

+

V r e f

1.5

0

Vref

1

p u

PIparameters

45

0

Pgain2

1.4

10

0

Tconst2

0.35

*57.29578

G1 + sT1

1 + sT2





47

Fig. 4 Generation of Reference Sine Wave

Fig. 5 PWM Carrier Signal

The sinusoidal pulse width technique is used to generate thefiring pulse of GTO’s so as to mitigate the voltage sag

condition. This technique is shown in Fig. 4 and Fig. 5. In thistechnique, PWM carrier signal are compared with thereference signal. Here the phase locked loop (PLL) plays an

important role in synchronizing the switching to the

distribution system voltage and lock to the phase atfundamental frequency to generate the PWM triangular carriersignals. When a three phase voltage is fed to the phase lockedloop, it generates a ramp signal which is then multiplied with

the switching frequency (set at 1.65 kHz, which is 33 times

the system operating frequency) so as to convert the rampsignal to a triangular signal whose amplitude is fixed between-1 to +1 which is shown in fig. 4. In Fig. 5, the three phase

voltages are applied to the phase locked loop (PLL) togenerate reference signal at wanted fundamental frequency.

5.

Simulation Results and DiscussionThe performance of the D- STATCOM model is evaluated bymeans of simulation techniques using the PSCAD/EMTDC

transient simulation program. The D- STATCOM is

connected to a 22 kV distribution system with a static load of5.2 MVA, 0.80 power factor. There are six single- phasetransformers with each rated at 1MVA, 22/4.16 kV and a

leakage reactance of 0.01 p.u connecting the D- STATCOMto the distribution system. Simulations are carried out toillustrate the effectiveness of the D- STATCOM for voltage

sag mitigation, as described below.

To illustrate the use of the D- STATCOM to compensatevoltage sags, a voltage sag condition is simulated by creating

Fig. 6(a) Voltage (pu), Real Power (MW) and Reactive

Power (MVAR) under Voltage Sag condition

a balanced three phase to ground fault component available inPSCAD/EMTDC software library. Simulation results showing

the use of D- STATCOM for compensating voltage sags are

shown in Fig. 7. in terms of the load voltage in per unit. For

the system without D- STATCOM, the load voltage dropsfrom 1.0 p.u to 0.7 p.u as shown in Fig.6(a). This is a voltagesag condition which is due to a three- phase fault created attime t = 1.5s for a duration 0f 0.75 sec. For the system with

the D- STATCOM connected, the load voltage increases from0.7 p.u to 0.9 p.u as shown in Fig.6(b). The load voltage

almost returns to its rated value due to the voltage sagcompensation capability of the D- STATCOM. Fig. 6(b) also

shows that the load ripple voltage at the starting and theending of compensation are due to the charging anddischarging of the D- STATCOM capacitor.

Va

Vb

Vc

PLLSix

Pulse6

thetaY

Ec

Eb

Ea

4

5

6

1

2

3

1

2

3

4

5

6

Shft

D+

F

-

30.0

RSgnOn

RSgnOff

GiPLL1GpPLL1

PLL ...

100

0

GpPLL1

50

PLL ...

1000

0

GiPLL1

500

Sin

Array6 6

Sin

Array6 6

Shift:

66

(in-sh)in

sh

Va

Vb

Vc

PLLtheta

Ec

Eb

Ea

*

1

2

3

4

5

6

1

2

3

4

5

6

TrgOn

TrgOff

PWM Setting1

Modulo

360.0

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

1.10

1.20

y

Vrpu

Main : Graphs

Time ... 0.00 0.50 1.00 1.50 2.00 2.50 3.00

-3.5k

-3.0k

-2.5k

-2.0k

-1.5k

-1.0k

-0.5k

0.0

0.5k

1.0k

1.5k

R e a c t i v e P o w e r ( M V A R )

Qm

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Time(... 0.00 0.50 1.00 1.50 2.00 2.50 3.00

-1.50

-1.00

-0.50

0.00

0.50

1.00

1.50

2.00

V o l t a g e ( p u )

Vrpu





48

Fig. 7(b) Voltage (pu), Three phase Voltage (kV), Real

Power (kW) and Reactive Power using D- STATCOM

6. CONCLUSION

The mitigation of voltage sag problems have been

investigated by using D- STATCOM. A simulation model ofthe 12- pulse D- STATCOM has been designed using the

PSCAD/EMTDC software and a SPWM- based controlscheme has been implemented to control the GTO’s of the

inverters. The GTO’s are connected inversely and parallel tothe diodes for commutation purposes and to charge thecapacitor. GTO’s are used in this simulation because it is easy

to control the switch on and off their gates and suitable fordesigned the D- STATCOM. From the simulation results, it isclear that D- STATCOM works well in mitigating voltage sagcaused by three phase to ground fault

7. REFERENCES[1] Hendri Masdi, Norman Mariun and S.M. Bashi, “Design

of a Prototype D- STATCOM for voltage sag mitigation”European Journal of Scientific Research, ISSN 1450-

216X Vol.30 No.1 (2009), pp.112-127 © EuroJournalsPublishing, Inc. 2009

[2] D.K. TANTI1, BRIJESH SINGH2, DR. M.K. VERMA3,

DR. O. N. MEHROTRA “An ANN Based Approach for

optimal placement of DSTATCOM for Voltage Sag

mitigation” International Journal of Engineering Scienceand Technology (IJEST)

[3] S. Asha Kiranmai, M.Manjula and A.V.R.S.Sarma

“Mitigation of Various Power Quality Problems UsingUnified Series Shunt Compensator in PSCAD/EMTDC”

16th NATIONAL POWER SYSTEMS CONFERENCE,15th-17th DECEMBER, 2010

[4] M. A. Hannan and Azah Mohamed, Member,IEEE

“PSCAD/EMTDC Simulation of Unified Series ShuntCompensator for Power Quality Improvement” IEEETRANSACTIONS ON POWER DELIVERY, VOL. 20, NO. 2, APRIL 2005

[5] Bhim Singh, Kamal Al-Haddad, Senior Member,IEEE,

and Ambrish Chandra, Member, IEEE “A Review ofActive Filters for Power Quality Improvement” IEEE

TRANSACTIONS ON INDUSTRIAL ELECTRONICS,VOL. 46, NO. 5, OCTOBER 1999

[6] M A Hannan and A Mohamed, Senoir Member, IEEE.“Unified Series-Shunt Compensator: Modeling and

Simulation” National Power & Energy Conference(PECon) 2004 Proceedings, KuaIa Lumpur, Malaysia

[7] M.A. Hannan, A. Mohamed, A. Hussain and Majid alDabbay “Development of the Unified Series-ShuntCompensator for Power Quality Mitigation” AmericanJournal of Applied Sciences 6 (5): 978-986, 2009 ISSN

1546-9239 © 2009 Science Publications

[8] Olimpo Anaya-Lara and E. Acha, “Modeling andAnalysis of Custom Power Systems byPSCAD/EMTDC” IEEE TRANSACTIONS ON

POWER DELIVERY, VOL. 17, NO. 1, JANUARY

2002

[9] Manitoba HVDC Research Centre, “PSCAD/EMTDC-Electromagnetic transients program including dc

systems”, 1994.

[10] A. M. Gole, O. B. Nayak, T. S. Sidhu and M. S. Sachdev,“A graphical electromagnetic simulation laboratory for

power system engineering programs” IEEE Trans. Powersyst., vol. 11, pp. 599-606, May 1996.

[11] N. Hingorani, “FACTS— Flexible ac transmission

systems,” in Proc. IEE 5th Int. Conf. AC DC

Transmission, London, U.K., 1991, Conf. Pub. 345, pp.1 – 7.

[12] “Introducing custom power,” IEEE Spectrum, vol. 32,

pp. 41 – 48, June 1995.

[13] N.G. Hingorani, L. Gyugyi, “Understanding FACTSConcepts and Technology of Flexible AC Transmission

Systems”, IEEE Press, New York, 2000.

Main : Graphs

Time(... 0.00 0.50 1.00 1.50 2.00 2.50 3.00

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PSCAD Simulation Model of D-Statcom for Voltage Sag Improvement